The present invention relates to digital electronic key telephone switching systems and private branch exchanges (PBX's) and more particularly, to a station interface for a digital electronic telephone switching system having a centralized digital audio processor. The present invention relates particularly to electronic key telephone systems in which voice signals are converted to digital form before they are transmitted to the central switching unit, also known as a key service unit (KSU), for retransmission to other telephones in the system or to trunk lines.
In a telephone switching system, it is necessary to modify the gains of various audio signals. These gain levels are dependent on the circuit to which a particular telephone is connected. For example, it is normal to provide a different gain setting if a telephone is connected to a central office (C.O.) trunk or lines are compared to when it is connected to another telephone inside the same system. Also, for supervisory signals like ringing tones, it is normal to provide a means to control the level of the ringing, often in the form of a volume control potentiometer or a multiposition switch located at a particular telephone.
Traditionally these gain control functions have been done on a decentralized basis, i.e., the gain controlling elements are resident in the circuitry of the individual circuit, e.g., in the telephone or in a line or trunk circuit. Because of the cost associated with this individual gain adjustment circuitry, the number of gain adjustments are kept to a minimum, thus providing acceptable performance for most conditions but not optimum transmission performance.
Another application where gain modification of audio is used is in hands-free speaker phone applications, e.g. U.S. Pat. No. 3,860,756. In this mode of telephone operation, a microphone and speaker are used as the audio transducers and, in one embodiment, a voice switching circuit is used to switch the telephone into either the receiver or the transmitting mode, depending on whether the telephone user is either listening or talking. Such function is necessary for hands-free operation in order to prevent feedback from the speaker to the microphone.
The basic principle for voice switched operation is to be able to detect which side of the communication channel is transmitting the signal and switch the voice path accordingly. Normally, some parameter that is a function of signal power is sensed and acted upon.
Speaker-phone circuits tend to be fairly complex circuits because of the complex nature of the parameters that have to be considered in the design of the circuitry. Some parameters that affect speaker phone performance are ambient background noise, speech signal levels and speech echo. Because of the wide variation in these parameters and because it is necessary to respond very quickly and accurately to changes in Receive/Transmit path switchovers, the circuits can be complex. Normally the more parameters that a circuit can monitor and take into account in the determination of switching thresholds, the better is the switching performance. However, the complexity of a circuit is proportional to its cost. This practically limits the performance of speaker-phone circuits which are provided on an individual telephone basis.